High-impact polystyrene, known by its English acronym HIPS, is a translucent or white opaque material which results from the dispersion of an elastomer phase (usually polybutadiene (PB) with high to low addition content 1,4-cis) in polystyrene (PS). Thanks to the dispersion of quantities of PB varying from 5 to 14% w/w, the material obtained exhibits great impact resistance, which makes it useful for various applications in household electronic devices; in the construction industry; in the packing of foods, goods, and other items; and in other areas. (The percentage mentioned in the preceding sentence represents the percentage of PB in HIPS, wherein the units are weight-by-weight. For instance, xe2x80x9c8% w/wxe2x80x9d means that each 100 weight units of HIPS contains 8 weight units of PB and 92 weight units of PS.)
The conventional process for preparation of HIPS consists in polymerization of the styrene monomer in the presence of the appropriate quantity of PB. In this process the agitation must be carefully controlled within the conversion range from 10 to 40% w/w, where the discontinuous styrene-polystyrene phase becomes the continuous phase and the PB is dispersed as particles with an average size which varies from 1 to 5 xcexcm. (The percentage in the preceding sentence is the weight-to-weight percentage of styrene that is polymerized.) The particles are composed of partially crosslinked PB with subinclusions of PS and PS grafted to the PB (PB-graft-PS) located in the interfaces. Only a determinated amount of PB of the total amount of PB becomes crosslinked. The immiscibility of PB in PS and the in situ formation of the compatibilizer, PB-graft-PS, give rise to the formation of varied morphologies, for example, of the lamellar, globular, salami type, capsule, foam, and others. These particles are responsible for the absorption of energy when the material is subjected to high intensity (impact) or low intensity (tension) forces, thus avoiding the growth of crazes. (The term xe2x80x9ccrazesxe2x80x9d refers to a multitude of very small cracks which develop to produce failure.)
In contrast to the good impact properties, the dispersion of PB in PS results in materials with reduced optical properties due to the difference in the indices of refraction of the two polymers. This fact translates into the reduction of transparency (opacity) and low luster of the finished products. However, this disadvantage may be reduced if the average particle size is reduced to values less than 1 xcexcm and if the particle-size distribution range is narrowed.
On the other hand, according to reports found in the literature, at a constant concentration of PB, a reduction in particle size results in a gradual increase in the impact resistance due to the increase in the number of particles; however, there is a minimum particle size below which the impact resistance no longer improves or improves minimally. In this respect, the authors seem to be in agreement that the minimum value must be greater than the molecular dimensions, i.e., approximately 0.01 xcexcm. At this minimum size, it is possible to consider that the particles are adequately large to retain the elastomeric properties.
Although, to date there has been only an ambiguous pronouncement concerning the effect of particle size on the impact resistance, it is clear that the two parameters (i.e., particle size and impact resistance) cannot be correlated without taking other factors into account, such as particle-size distribution, morphology, interfacial adhesion, degree of crosslinking of the PB, volume of the PB phases, PB content, size of grafts, and possibly other factors.
Control of these parameters cannot be carried out due to the nonexistence of selective chemical reactions. In effect, the formation of the surface agent (PB-graft-PS), which, to a large measure, determines particle size and interfacial adhesion, cannot be controlled by means of the conventional HIPS preparation process. This is due, on the one hand, to the random nature of the reaction, and, on the other hand, to the lack of understanding of the exact mechanism associated with the grafting reaction and its kinetics. Simply stated, the grafting process, i.e., the process by means of which the PB is gradually converted into PB-graft-PS, may be divided into two steps (see Diagram 1). 
In the first step, the formation of a radical on the PB chain occurs and this is followed by the initiation and propagation reactions of styrene polymerization. The formation of polybutadiene radical PB. in any of its forms (1, 2, 3, 4 below) by abstraction or addition, is the determining factor in the rate of conversion of PB into PB-graft-PS. 
The formation of this grafted copolymer is limited by the low selectively of the primery radicals (RO.) as defined by Equation 1, in which kPB and kS are the reaction rate constants. [B] and [S] are the the molar concentrations of butadiene and styrene units, respectively. For example, in Equation 1, if [B]/[S] is about 0.1 and kS greater than kPB, then c less than 1.
C=(kPB/kS)[B]/[S]xe2x80x83xe2x80x83(Equation 1)
The invention includes a method for functionalizing a polymer, the method comprising reacting the polymer with a free radical of formula A to yield a functionalized polymer, wherein formula A is defined below. The polymer may be selected from the group consisting of polybutadienes with any microstructure, copplymers or terpolymers based on butadiene with any topology and composition, ethhylene-propylene-diene monomers and terpolymers (EPDM) with any topology and composition, polyisoprene or its copolymers with any topology and composition, polychloroprenes or copolymers of chloroprene with any topology and composition, polyvinyl chlorides or copolymers of vinyl chlorides, natural rubber regardless of its origin, alkyl polymethacrylates, alkyl polyacrylates, polyacetylenes, polyacrylonitriles, polyvinyl acetates, and mixtures thereof. (The term xe2x80x9ccompositionxe2x80x9d is well-known in the art to refer to the molar relation of monomers comprising the copolymer or terpolymer.) Other polymers are set forth below in the xe2x80x9cDescriptionxe2x80x9d section.
The invention also includes a method for preparing a copolymer or terpolymer, the method comprising initiating polymerization of a monomer or a mixture of monomers with a modified polymer to yield the copolymer or terpolymer, wherein the copolymer or terpolymer comprises two or more monomers.